http://www.zibri.org/ describes how to connect a Mattel Jedi Force Trainer to the pc via a TTL/RS232 interface.
„This is a Propeller laptop – with a 6502 co-processor and 64K of static RAM! The Propeller handles all I/O for the 6502 and runs an integrated debugger so you can program the computer. The Propeller serves as the programmable chipset for this 6502 laptop. You could use an FPGA in this capacity, but could you easily do this and implement visual debugger software inside an FPGA? Over the years hardware prototyping has evolved from building-block hardware (TTL) to programmable hardware (PLA’s and FPGA’s). I believe the Propeller represents the next revolution: 100% software-based virtual hardware – and I built this laptop to prove it!
I’ve always want to design a computer from scratch, but modern CPU’s are too complex, run too fast, and have too many fiddly little pins to be any use in a DIY project. An older processor like the 6502 is much easier to work with, yet you can still learn about computer architecture from it. If you think an 8 bit processor is irrelevant because it is old, consider that just because 8 bit technology is out of fashion, does not mean the principles of computing have changed. These principles, like mathematical concepts such as π and square roots, do not become obsolete just because they are old.“